Existentialism is a part of the general philosophical reaction to WW2, evil in the world, impossibility of knowing anything for sure and of expressing anything exactly, and the realization that mankind invents everything… that existence precedes essence. Also, disbelief in gods, lots of awareness of awareness, and the realization of the ultimate pointlessness of everything… being alive, being a human, everything has no non-human point to it.
Existentialism does posit human responsibility for our world as it is and for our own selves, but nevertheless, we do not have a rule book of goodness/badness. Thus we are trapped in a senseless world that we nevertheless make sense of and function as if we have it all solved.
Existentialism led to post modernism which gives up on responsibility and admits nothing means nothing, we make up everything, and we can remain partly sane only by adopting an ironic stance toward everything.